🕳️ Stress Concentration

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · Checked against 1 independent reference calculation · how PhDino checks its numbers

Related standards: AISC 360

How a geometric discontinuity like a hole or notch locally raises stress well above the nominal average.

Away from any holes, notches, or sharp corners, stress in a loaded member is roughly uniform across the section — the "nominal" stress. Right next to a discontinuity, though, stress lines have to crowd around it, and the local peak stress can be several times higher than the nominal value. The stress concentration factor Kt captures that ratio for a given geometry.

Kt values for common geometries (holes, fillets, notches, shoulders) are tabulated in references like Peterson's Stress Concentration Factors, as a function of the geometry ratios involved (e.g. hole diameter to plate width).

Key formula

σ_peak = Kt × σ_nominal

Variables

Kt

stress concentration factor (geometry-dependent, always ≥ 1)

σ_nominal

average stress on the net section, ignoring the discontinuity

Notes & limitations

• Kt describes elastic behavior; if the peak stress exceeds yield, local yielding blunts the actual peak and Kt overestimates the true stress — but for fatigue, the elastic Kt is usually still the right factor to use, since fatigue cracks initiate from elastic-range stress cycling.
• This is exactly why fatigue failures so often start at holes, keyways, fillets, and threads — geometry, not raw load, is frequently the real culprit.